Device to Device (D2D) communication may refer to a communication method in a wireless communication system for a user equipment (UE) to directly communicate with another UE. Conventionally, whenever a UE transmits a message to another, a base station or an evolved Node B (eNB) in the case of LTE would relay the message in between the two UEs. But for the D2D communication, the involvement of the eNB has been minimized as user messages of the two UEs could be delivered directly from the one UE to the other.
However, in order to for D2D devices to be synchronized to a network or to another peer device, D2D synchronization information may be delivered either from the network or from a peer device to each individual UE. Hence, a synchronization source (SS) could be utilized to transmit a D2D synchronization signal (D2DSS) to enhance the network synchronization. When a network implements synchronous D2D communication as opposed to asynchronous D2D communication in which a device would blindly search for another device, all D2D devices could be required to be aligned to the network timing according to a synchronization signal.
Therefore, at least two different types of apparatuses could serve as a synchronization source. Conventionally, an eNB may serve as a synchronization source by periodically transmitting system information in which a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) are located. When the PSS/SSS have been received, a D2D UE may align its timing with the timing of the eNB. Also, another peer device could also serve as a synchronization source. In particular, any D2D UE may transmit a D2DSS that can be received by another D2D UE as a synchronization source.
In order to design a D2D synchronization scheme, several issues would need to be considered. First, a D2D device may simultaneously receive several synchronization signals at once. In that case, a D2D device would be considered with whom the D2D device would synchronize itself to. If the D2D device chooses the wrong device to follow, the timing of the D2D device could be misaligned. Also, there could be a timing confusion in the case when a D2D device receives two or more timing references that are different. In that case, the D2D device would not know which timing reference it should follow. Secondly, after a D2D device selects a timing reference, the D2D device may not necessarily have to forward the timing reference as selected.
In any case, a D2D synchronization method could be designed to ensure the reliability of network timing and to minimize potential timing confusions.